1. Field of the Invention
This invention relates to a hearing aid.
2. Prior Art
The hearing aid is so designed that outside sounds, such as the voice, are converted by a microphone into electrical signals which are amplified by an amplifier and supplied to an earphone after correction of the frequency characteristics thereof in accordance with the characteristics of the user's auditory sense, and the thus corrected voice output is supplied from the earphone to the user's external auditory meatus.
Most of the prior-art hearing aids are of the separate type consisting of a main body provided with a microphone unit and a earphone section separate from the main body. However, the separate type hearing aids are not in popular use because they are difficult to handle and cannot be used without being noticed by a third person. Recently, with miniaturization of components, brought about with the progress in the production technology, a unitary type hearing aid, in which the microphone unit, earphone unit, amplifier and the storage battery are accommodated in a main body, has been presented to the market.
In general, the hearing aid is attached to the user's auricle, with the earphone stopping up the external auditory meatus. The user of the hearing aid may hear the sound output emanating from the attached earphone unit as the voice by such sound output reaching the tympanic membrane by way of the external auditory meatus and vibrating the tympanic membrane.
Since the conventional hearing aid is attached to the user's auricle, with the earphone stopping up the external auditory meatus, the sound signal reaching the user's tympanic membrane is reflected thereat to return to the earphone unit by way of the external auditory meatus so as to be reflected by the diaphragm or the housing of the earphone unit to proceed again towards the tympanic membrane. In this manner, both the sound directly emanating from the earphone unit and the sound emanating from the earphone unit and reflected by the tympanic membrane and the earphone unit reach the tympanic membrane. With such conventional hearing aid, in which both the sound directly emanating from the earphone unit and the reflected sound reach the user's tympanic membrane, the user may be fatigued as he feels oppressed or as if the sound source were located within his or her own head. In addition, because of the presence of the reflected sound, it has been difficult to carry out a suitable correcting operation for the voice output by an electrical circuit.
On the other hand, the unitary type hearing aid is liable to howling and unstable operation, because the microphone unit and the earphone unit are mounted close to each other. Besides, the outside noise tends to intrude so as to render it difficult for the user to hear the desired sound.